Tectonic and Seismic Implications of an Intersegment Rupture the Damaging May 11Th 2011 Mw 5.2 Lorca, Spain, Earthquake

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Tectonic and Seismic Implications of an Intersegment Rupture the Damaging May 11Th 2011 Mw 5.2 Lorca, Spain, Earthquake Tectonophysics 546-547 (2012) 28–37 Contents lists available at SciVerse ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Tectonic and seismic implications of an intersegment rupture The damaging May 11th 2011 Mw 5.2 Lorca, Spain, earthquake José J. Martínez-Díaz a,⁎, Marta Bejar-Pizarro b, José A. Álvarez-Gómez a,c, Flor de Lis Mancilla d,e, Daniel Stich d,e, Gerardo Herrera b, Jose Morales d,e a Dpto. de Geodinamica, Universidad Complutense, IGEO (UCM-CSIC), Calle Jose A, Novais 2, 28040 Madrid, Spain b InSARlab Geohazards InSAR laboratory Geohazards group Geoscience Research dept., Geological Survey of Spain C/Alenza 1 28003 Spain c Instituto de Hidráulica Ambiental “IH Cantabria”, Universidad de Cantabria, E.T.S.I. Caminos, Canales y Puertos, Santander, Spain d Instituto Andaluz de Geofísica, Campus Universitario de Cartuja, Universidad de Granada, Granada, Spain e Departamento de Fisica Teorica y del Cosmos, Universidad de Granada, Granada, Spain article info abstract Article history: On May 11th 2011, a Mw 5.2 earthquake stroke the city of Lorca in the SE Spain. This event caused 9 fatalities, Received 9 February 2012 300 injuries and serious damage on the city and the surrounding areas. The Lorca earthquake occurred in the Received in revised form 5 April 2012 vicinity of a region bounding two well-known segments of a large active fault, the Alhama de Murcia fault Accepted 11 April 2012 (AMF). The Lorca earthquake offers a unique opportunity to study how strain is accommodated in an inter- Available online 24 April 2012 segment region of a large strike slip fault. We map recent tectonic structures in the epicentral region and we use radar interferometry to analyze the coseismic deformation. Combining these data with seismological ob- Keywords: fi fl Betic Cordillera servations of Lorca seismic sequence we rst model the source of the earthquake. Then we analyze the in u- INSAR ence of our preferred model in the adjacent segments by Coulomb failure stress modeling. The proposed Coulomb stress transfer earthquake source model suggests that this event ruptured an area of ~4×3 km within the complex structure Seismic hazard that limits the Goñar–Lorca and Lorca–Totana segments of the AMF. The induced static stress change on the Active tectonics adjacent segments of the fault represents a seismic cycle advance equivalent to 200 to 1000 years of tectonic Intersegment zone loading. © 2012 Elsevier B.V. All rights reserved. 1. Introduction zones behave as relaxation barriers and large ruptures skip over the intersegment area that remains unbroken (Das and Aki, 1977; Active faults are organized in more or less uniform segments sep- Scholz, 1990). The study of small earthquakes that break these in- arated by intersegment regions, characterized either by a change in tersegment areas gives us the opportunity to analyze this complex the geometry of the fault or by the presence of structural complexities behavior. (Elliott et al., 2012; Fliss et al., 2005; Harris and Day, 1993; King, On May 11th 2011, a Mw 5.2 earthquake stroke the city of Lorca in 1986; Klinger, 2010; Shengji et al., 2011; Wesnousky, 2006). Segmen- south-eastern Spain. This earthquake occurred 2 h after a Mw 4.6 tation is important because of its implications for the rupture behav- foreshock and caused 9 fatalities, 300 injuries, serious damage on ior during earthquakes. Segments are fault slip prone areas during 1164 buildings and economic losses over 1200 M€ (data from the large earthquakes (DePolo et al., 1989; 1991), whereas intersegment Municipality of Lorca updated November 2011). The Lorca earth- zones are defined as areas where rupture begins or stops during an quake is especially significant because it occurred in the vicinity of a earthquake (e.g. Aochi et al., 2000; Jackson et al., 2006; Klinger region bounding two well-known segments of a large active fault, et al., 2005; Lozos et al., 2011). This complex behavior is observed the Alhama de Murcia fault (AMF) (Fig. 1). This fault is the source not only in the horizontal rupture propagation, but also in the rupture of Mw>6.5 historical and pre-historical earthquakes (Martinez-Diaz propagation at depth (Elliott et al., 2011; Jackson et al., 2006; Li et al., et al., 2001). The AMF accommodates ~0.1–0.6 mm/year of the 2011; Nissen et al., 2010). During major and less frequent earth- approximately 5 mm/year of convergence between African and quakes, several segments can slip at a time, whereas the intersegment Eurasian plates (Masana et al., 2004) and belongs to the Eastern zones can behave as areas of high slip release. This occurred during Betics Shear Zone (Silva et al., 1993). The AMF is one of the largest the Wenchuan earthquake (Shen et al., 2009). In other cases these faults of this shear zone. Most of the largest damaging historical earthquakes are related with this structure (Fig. 1). This fault presents aNE–SW direction; it is ~100 km length and is divided into 4 seg- ⁎ Corresponding author at: Calle Jose A, Novais 2, 28040 Madrid, Spain. Tel.: +34 – – – 913944835; fax: +34 913944634. ments: Goñar Lorca; Lorca Totana; Totana Alhama de Murcia and E-mail address: [email protected] (J.J. Martínez-Díaz). Alhama de Murcia–Alcantarilla (Fig. 1B). Paleoseismic studies along 0040-1951/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2012.04.010 J.J. Martínez-Díaz et al. / Tectonophysics 546-547 (2012) 28–37 29 4° W 2° W 0° Iberian peninsula M editerranean sea 39°N cordillera Betic Tell Rif African plate BSF 38°N CF Intensity (EMS) VI PF 37°N VII VIII AMF IX CF X 36°N A 050 100 200 Kilometers M Murcia Alcantarilla IAG IGN HAR Fortuna basin EXAH A-A N F Alhama Espuña Range EMUR IAG de Murcia Carrascoy Range EXZA 37°50' VALD Totana Lorca basin T- A S T Gualdalentín valley Lorca L-T Las Estancias UMV6 N Range 37°35’ 37°35’ VELZ Pto. Lumbreras MAZA G-L S E Quaternary deposits l Neogene deposits B 0 10 20 Km 2° W 1° 45'W 1° 30'W 1° 15'W Fig. 1. A) Location map of the study area in which the Quaternary active faults are projected. Circles represent the historical seismicity with intensity (EMS)>VI (data from the Instituto Geografico Nacional). The ellipse indicates the position of the Eastern Betic Shear Zone, CF: Carboneras fault; PF: Palomares fault; AMF: Alhama de Murcia fault; CCF: Carrascoy fault; BSF: Bajo Segura fault. B) Map of the Alhama de Murcia fault, arrows indicates the limits of the four main segments of this fault: GL: Goñar–Lorca segment, LT: Lorca–Totana Segment, TA: Totana–Alhama segment; AA: Alhama–Alcantarilla segment. ST: Sierra de La Tercia. The star and the circles are the mainshock and aftershocks of the Lorca 2011 seismic sequence taken from Lopez-Comino et al. (2012). Focal solutions of the foreshock (F) and the mainshock (M) from several agencies are shown, IAG: Instituto Andaluz de Geofisica; IGN: Instituto Geografico Nacional; HARV: Harvard University. In both maps the black triangles are the seismic stations utilized in the aftershock relocation. this fault suggest that the two segments converging on Lorca (Goñar– The Lorca earthquake offers a unique opportunity to study how Lorca and Lorca–Totana) ruptured during the Quaternary as a single a strain is accommodated in this region that includes not only the seismogenic source producing earthquakes of Mw 6.9–7.3 (Masana AMF but also secondary active structures. We map recent tectonic et al., 2005; Ortuño et al., 2012). Other paleoearthquakes identified structures in the epicentral region and we use radar interferom- on these segments are smaller (Mw~6) and seem to have ruptured etry to constrain the coseismic deformation. Combining these data only one segment (Masana et al., 2004). The Lorca intersegment with published seismological data of the seismic sequence of Lorca area could play a significant role in the seismogenic behavior of the (Lopez-Comino et al., 2012) we model the earthquake source. Using AMF. Coulomb Failure Stress transfer (ΔCFS) models we analyze the 30 J.J. Martínez-Díaz et al. / Tectonophysics 546-547 (2012) 28–37 influence of our preferred source in the adjacent segments. We finally the other nodal plane perpendicular to the AMF dipping to the SW compare our results with topography and fault structure close to the (IAG, 2011; IGN, 2011)(Fig. 2). The latter is difficult to explain from rupture area and discuss the implications for strain accommodation, the local structure. The former, on the other hand, is parallel to sev- fault behavior and seismic hazards in the region. eral branches of the AMF in the intersegment zone. Aftershocks regis- tered until the 7th July were relocated by Lopez-Comino et al. (2012) using a dense seismic station network (Fig. 2). The aftershock epi- 2. Structure of the epicentral area centers aligned parallel to the AMF and concentrated on the north of the intersegment zone. These evidences suggest that the source The Lorca 2011 earthquake occurred near the intersegment of the earthquake is parallel to the AMF dipping to the north as pro- zone located between Goñar–Lorca and Lorca–Totana segments posed in a preliminary study by Vissers and Meijninger (2011).In (Fig. 1). A field survey conducted in the epicentral area 2 days this work we use InSAR measurements of the coseismic deformation after the earthquake concluded that the Lorca earthquake did not to better define the earthquake source parameters. rupture the surface (IGME, 2011).Weperformeddetailedmapping of recent structures on the epicentral area to understand the kine- matic of structures in the intersegment zone and to assess the 3.
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